Each of the 175 or so planes in question has taken off and landed more than 30,000 times and will receive repetitive electromagnetic inspections at regular intervals for fatigue damage.

Planes with less than 30,000 total flight cycles can wait until they reach that level to be inspected, the directive said.

In a separate statement, Boeing said it was working with the FAA, the National Transportation Safety Board and Southwest, and it was preparing a service bulletin to recommend lap joint inspections on certain 737 models.

NTSB board member Robert Sumwalt said Monday that inspectors have visually checked lap joints in the past but haven't used high-tech monitoring because no one thought that part of the plane was susceptible to cracks. Moving forward, inspectors will test the joints with an electromagnetic process.

The chairman on the U.S. Senate Committee on Commerce, Science and Transportation said in a statement that the American public wants answers to retain confidence in commercial air travel.

"As the details of this unusual incident unfold, we will get a better understanding of what caused the plane's fuselage to tear open," said Sen. Jay Rockefeller, D-West Virginia.

"I expect the FAA to be painstakingly diligent in reviewing the safety of all aircraft, and to conduct a careful investigation into what caused the cracks that have been discovered on the bodies of these planes."

After Southwest canceled about 600 flights over the weekend to accommodate inspections, the airline said it canceled another 70 flights Monday.

Whitney Eichinger, a spokeswoman for Southwest, said the airline finished its inspections of its fleet of 79 737-300s on Monday night. Five aircraft were found to have subsurface cracking and will need repairs, she said.

The situation will not have any impact on Southwest's flight schedule Tuesday.

Eichinger said all Southwest aircraft that would be covered by the FAA inspection directive have been perused.

She could not say how long it will take to repair the five aircraft and get them back in service. The company is waiting for recommendations from Boeing on how to address the problem.

Eighteen minutes into Southwest Flight 812 from Phoenix to Sacramento, California, on Friday, a hole five feet long and one foot wide opened in the fuselage, causing the cabin to lose pressure, the NTSB said.

One flight attendant suffered minor injuries, the agency said.

The pilot initially planned to return to Phoenix, but he made an emergency landing at a military base in Yuma, Arizona, after flight attendants reported seeing blue sky through the jet's roof, Sumwalt told reporters.

Sumwalt said the 737 used for Flight 812 had a maintenance check in March at Southwest's Dallas facility, and the jet had no outstanding maintenance issues at the time of the accident, with all its records "positive, up to date, (and) with no discrepancies."

The plane's flight data recorders had no noteworthy information on them, Sumwalt said.